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Bureau of Mines Information Circular/1985 



Evaluation of Sensitive Ground Fault 
Interrupters for Coal Mines 

By Michael R. Yenchek and Melvin N. Ackerman 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75 

AflNES 75TH AV*^ 



Information Circular 9057 

(f 

Evaluation of Sensitive Ground Fault 
Interrupters for Coal Mines 

By Michael R. Yenchek and Melvin N. Ackerman 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodei, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




m.9e>s7 



Library of Congress Cataloging in Publication Data; 



Yenchek, M. R. (Michael R.) 

Evaluation of sensitive ground fault interrupters for coal mines. 

(Information circular / United States Department of the Interior, 
Bureau of Mines ; 9057) 

Bibliography: p. 15. 

Supt. of Docs, no.: I 28.27: 9057. 

1. Coal mines and mining— Electric equipment. 2. Electric cir- 
cuit-breakers. I. Ackerman, Melvin N, II, Title. III. Series: Infor- 
mation circular (United States. Bureau of Mines) ; 9057. 



TN295.U4 [TN343] 622s [629'. 8] 85-600192 



■A 

"j^ CONTENTS 

Page 

Q Abstract. 1 

"13 Introduction 2 

N^ Ground fault protection in U.S. mines 2 

^^Description of evaluated ground fault interrupters 3 

\. General Electric ground break relay 3 

V) GBS Harrison ground fault detector. 4 

Mindel ground fault circuit interrupter 4 

Test rationales and results,... 5 

Proper design and construction , 5 

Applicable military standards 5 

Mineworthiness 6 

Size limitations 6 

Electrocution prevention 7 

Tests at 60 Hz 7 

Power harmonics 8 

Transient immunity 10 

Voltage surges 10 

Common mode transients 12 

Current withstand 13 

Reliability 13 

Quality assurance 13 

Safe failure modes 14 

Summary and conclusions 14 

References 15 

ILLUSTRATIONS 

1 . Zero-sequence relaying 2 

2. General Electric type MC ground break relay 3 

3. GBS Harrison type GF2B ground fault detector 4 

4. Mindel model 21-7000 Shok-Blok ground fault circuit interrupter (older 

version) 5 

5. Mindel model 21-7000 Shok-Blok ground fault circuit interrupter (newer 

version) 6 

6. GFI dimensions 7 

7. Ventricular fibrillation threshold at 60 Hz 8 

8. Test setup to determine tripping characteristics at 60 Hz 8 

9 . GFI response at 60 Hz 8 

10. GFI frequency response 10 

11. Frequency-response test circuit 10 

12. Basic impulse generator circuit 12 

13. Relay control circuit for impulse generator 12 

14. Current withstand test circuit 13 

>S TABLES 

.C^J^l. General Electric: Tripping characteristics at 60 Hz 9 



jvN2. GBS Harrison: Tripping characteristics at 60 Hz 9 

3. Older Mindel: Tripping characteristics at 60 Hz 9 

4. Newer Mindel: Tripping characteristics at 60 Hz 9 

5. General Electric: Frequency response data 11 



ii 



TABLES — Continued 



6. GBS Harrison: Frequency response data. 

7. Older Mindel: Frequency response data. 

8. Newer Mindel: Frequency response data. 

9. Winding resistance 

10. Current ratio tests 

11. Summary of results 



Page 



11 


11 


11 


13 


13 


14 





UNIT OF MEASURE 


ABBREVIATIONS USED IN 


THIS REPORT 


A 


ampere 


ms 


millisecond 


°C 


degree Celsius 


ys 


microsecond 


Hz 


hertz 


mV 


millivolt 


h 


hour 


fi 


ohm 


in 


inch 


s 


second 


kHz 


kilohertz 


V 


volt 


kfi 


kilohm 


VA 


volt ampere 


kV 
lb 


kilovolt 
pound 


V ac 


volt, alternating 
current 


mA 


milliampere 


W 


watt 


PF 


microfarad 


yr 


year 


UH 


microhenry 







EVALUATION OF SENSITIVE GROUND FAULT INTERRUPTERS FOR COAL MINES 

By Michael R. Yenchek and Melvin N. Ackerman 



ABSTRACT 

Contacts with energized conductors are a major cause of electrocutions 
in underground coal mines. Sensitive ground fault interrupters (GFI's) 
installed on in-mine three-phase ac utilization circuits would probably 
prevent the majority of these deaths. A sensitive GFI is a protective 
device that detects and interrupts small deadly ground currents in the 
milliampere range before those currents can cause ventricular fibrilla- 
tion in humans. Commercially available three-phase sensitive GFI's have 
not been specifically designed for application in coal mines. The Bu- 
reau of Mines therefore tested three commercial GFI models to determine 
their worthiness for mine power systems. GFI design and construction, 
transient immunity, reliability, and time-current characteristics were 
evaluated in laboratory tests. No commercial device was found suitable 
for mine use without design modifications. The tests results will serve 
as a basis for the development of a mineworthy sensitive GFI in ongoing 
Bureau research. 



Electrical engineer. 
^Electrical engineering technician (retired). 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



In a study completed in 1981, 307 sepa- 
rate accidents in the coal industry dur- 
ing a 3-year period were attributed to 
electric shock, from contacts with ener- 
gized conductors (J^).-^ These accidents 
resulted in seven fatalities and the loss 
of over 5,000 person-days from work due 
to nonfatal injuries. The majority of 
the nonfatal injuries and nearly all the 
electrocutions could have been eliminated 
if ground fault protection, designed to 
protect people, had been installed on the 
coal mines' resistance-grounded systems. 

The grounded phase protective devices 
presently used on these power systems are 
inadequate from a shock-prevention stand- 
point. Typical relay current pickup or 
response levels are in the ampere range, 
considerably in excess of the electrocu- 
tion threshold. Increasing the sensitiv- 
ity of these electromechanical devices 



results in undesirable nuisance tripping 
and unscheduled interruptions of coal 
production. What is needed is a sensi- 
tive GFI that identifies and inter- 
rupts the small deadly ground currents 
that can electrocute people, yet ignores 
spurious signals such as those from motor 
startups. 

Criteria have been established for the 
use of sensitive GFI's on low-voltage 
ac utilization circuits in U.S. mines 
(2). These practical guidelines include 
specific recommendations concerning GFI 
design and construction, transient immu- 
nity, reliability, and time-current char- 
acteristics. This report documents tests 
conducted by the Bureau of Mines in ac- 
cordance with these criteria using com- 
mercially available three-phase sensitive 
GFI's. 



GROUND FAULT PROTECTION IN U.S. MINES 



The majority of U.S. mine power systems 
employ radial distribution, wherein the 
supply power branches out through switch- 
houses and terminates at utilization 
points throughout the mine. The utili- 
zation system includes power centers, 
rectifiers, cables, motors, and the asso- 
ciated protective devices. It is the 
most troublesome part of the power sys- 
tem in terms of safety and reliability 
due to its temporary nature. As mining 
advances, the utilization system is 
stretched to its limit and then reposi- 
tioned. Thus, the circuit protective de- 
vices must adapt to constantly changing 
conditions . 

Typical ground fault protection in min- 
ing consists of high-resistance grounding 
and ground fault protective relaying. 
The resistance Inserted between the sys- 
tem neutral and ground limits the fault 
current and energy dissipation. The re- 
lay monitors the circuit and removes 

— > ■ — — — 

-•Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



power upon indication of a hazardous 
current flow. In present U.S. mining 
systems, the current permitted, and the 
current required to operate electro- 
mechanical relays , can create a personnel 
hazard before power is removed. 

Zero-sequence or balanced-flux relay- 
ing (fig. 1) is the most reliable and 
most common method employed for ground 
fault relaying. As shown in figure 1, 
the phase conductors pass through the 



Phase 



Current 
transformer 



B 
C- 




Zero- 
sequence 

relay 



Ground wire 



FIGURE 1. - Zero-sequence relaying. 



current transformer (CT) window. The sum 
of the three phase currents is the CT 
primary current and is proportional to 
the zero-sequence current O) . In an 
unfaulted balanced system, there is lit- 
tle or no zero-sequence current, and the 
CT secondary current is approximately 
zero. However, when a ground fault 



occurs , the resultant secondary current 
is used to trip a relay. Zero-sequence 
relaying is unaffected by phase voltage 
fluctuations, and, since only the ground 
leakage current is monitored, the relay 
can be made very sensitive. All of the 
commercial GFI's evaluated in this paper 
were the zero-sequence type. 



DESCRIPTION OF EVALUATED GROUND FAULT INTERRUPTERS 



Tests were conducted using three com- 
mercial sensitive GFI's identified under 
Bureau contract JO 113009 (2^) as having 
potential applicability to mining. 



GENERAL ELECTRIC GROUND BREAK RELAY 

The General Electric Co. (GE) type 
MC ground break relay (fig. 2) was 





I I I I I I I I I I I I I I I 



FIGURE 2. - General Electric type MC ground break relay. 



specifically designed to protect mo- 
tor circuits from ground faults. The 
TMCGS200GT current sensor provides a min- 
imum tap setting or pickup of 100 mA. 
Its rated response time is 300 ms at 150% 
of trip level. The relay is equipped 
with a button for resetting following 
clearance of a ground fault and a button 
to test relay operation, 

GBS HARRISON GROUND FAULT DETECTOR 

The GBS Harrison Ltd. type GF2B ground 
fault detector (fig. 3) was developed by 
the National Coal Board for coal mines in 
the United Kingdom, It has a rated sen- 
sitivity of 90 mA ±20% at 20° C and a 
rated trip speed of less than 100 ms at 
150% of trip current. A small light- 
emitting diode (LED) indicates the pres- 
ence of control power to the unit. A re- 
mote pushbutton can be added for periodic 
testing. 



MINDEL GROUND FAULT CIRCUIT INTERRUPTER 

The Mindel Corp.'s model 21-7000 Shok- 
Blok ground fault circuit interrupter 
(figs. 4-5) was originally designed by 
Thomas Gross, a consulting engineer and 
holder of patents for several ground 
fault detection techniques. The Mindel 
relay has been used by the irrigation 
industry, but only recently has been mar- 
keted for use in coal mines. The first 
units tested (fig. 4) were housed in 
plastic boxes with a test button on the 
front. Some featured a knob to adjust 
sensitivity from 20 to 100 mA. Following 
failure of two original units, a newer 
version of the model 21-7000 was also 
tested (fig. 5). It consisted of a plug- 
in module with test and reset buttons. 
It had a rated sensitivity of 60 mA with 
a rated maximum response time of 3 s. 





l^^-A.,4. 



GBS HARRISON LTD 



GROUND FAULT DETECTOR 
TYPE Gf2B 



POWER 



(SfNSlTIVITV 80-tOOM*) M*Of IN tUGLAND 

(120V AC i _^_ _^ fCOMTAC-S JSa VAC SAMP) 



FIGURE 3. - GBS Harrison type GF2B ground fault detector. 



lOOO 



ftOO*' 









e»»***" 




FIGURE 4. - Mindel model 21-7000 Shok-Blok ground fault circuit interrupter (older version). 



TEST RATIONALES AND RESULTS 



PROPER DESIGN AND CONSTRUCTION 

Applicable Military Standards 

Rationale 

Proper design and construction will re- 
duce the amount of downtime caused by GFI 
failures and thereby help bring about ac- 
ceptance of the GFI as a useful safety 
item. Electronic instruments designed 
and constructed for military use must 
comply with Military Standard 454 ( 4_) . 
The following two summarized portions of 
that standard can be applied to GFI's 
used in underground mining: 

Safety Hazard 

The design shall incorporate methods to 
protect personnel from accidental contact 
with voltages in excess of 30 V RMS or dc 
during normal operation. All external 



surfaces shall be at ground potential 
during normal operation. All terminals 
shall be corrosion-resistant. Sharp ex- 
ternal projections shall be avoided. 

Accessibility 

Suitable access shall be provided for 
adjustments, testing, and routine main- 
tenance. No unsoldering shall be nec- 
essary to remove the front cover for 
troubleshooting. 

Findings 

All of the relays were housed in plas- 
tic cases, and internal adjustments were 
unnecessary in any of them. Only the 
older Mindel relay did not have exposed 
terminals energized at 120 V ac. Access 
is required to change a fuse in this 
relay, but the front case is easily 
removable without unsoldering. The relay 





FIGURE 5. - Mindel model 21-7000 Shok-Blok ground fault circuit interrupter (newer version). 



components of the newer Mindel relay were 
mounted in a convenient plug-in module. 

The terminals on both the GE and Harri- 
son relays corroded badly when coated 
with acetic acid for 24 h. 

Mineworthiness 

Rationale 

Underground, GFI's are located inside 
metal-clad load centers, so both the re- 
lay and CT should have metal mounting 
lugs. Terminal strips should be sized 
for No. 12 AWG wire. In addition, the 
relay case should be moisture- and dust- 
resistant. 

Findings 

Only the Harrison GFI was equipped with 
metal lugs on both the CT and relay. 
However, it was also the only model with 
undersize wire terminals. 



All relays were housed in moisture- and 
dust-resistant cases. 

Size Limitations 

Rationale 

Space is limited in typical mine power 
equipment. Since several GFI's may be 
used in a single power center, they must 
not be much larger than present ground 
fault relays. Thus, the relay components 
should be mounted in a compact enclosure 
not exceeding 3 by 6 by 6 in. 

To minimize leakage flux, the CT window 
should only be large enough to accommo- 
date the encircled power conductors. 
Trailing cable size is limited to No. 4/0 
AWG to facilitate handling underground. 
The outside diameter of a 4/0 single con- 
ductor cable is 0.807 in O ) . Three such 
cables fit snugly through a 1 .750-in-diam 
windows. For ease of installation of 



cables with terminals, this value should 
be increased to 2.100 in. 

Present ground fault CT's in use under- 
ground have outside diameters smaller 
than 4 in. Since they are placed between 
the molded-case circuit breaker and the 
load-center coupler, they are no more 
than 3 in wide. 

Findings 

The dimensions of the CT's and relays 
are given in figure 6. All of the CT's 
have undersized windows, and only the 
outside diameter of the GE CT did not ex- 
ceed 4 in. All relay enclosures except 
the older Mindel were appropriately 
sized. 



CURRENT TRANSFORMER 



RELAY 




DO 



/a 



U|5/8-^' 



3'/-;' 



General Electric 




\-^x^ 



n 



■ — X^A 1^/4 h 



Harrison 




=0^ 



VA 





© 


6 


/4 


1 1 1 1 I M 1 








1 43/4" 







-4 'A" 




Mindel (older version) 

J nn n 







h-2VH 



Mindel (newer version) 
FIGURE 6. - GFI dimensions. 



ELECTROCUTION PREVENTION 

Tests at 60 Hz 

Rationale 

The primary reason for employing sen- 
sitive GFI' 8 in mining is to prevent ac- 
cidental electrocutions. The resultant 
cause of death in these instances is ven- 
tricular fibrillation. In this condi- 
tion, the normal heartbeat stops, and the 
ventricles twitch irregularly. The 60-Hz 
threshold has been statistically defined 
as the current through the chest that 
will produce ventricular fibrillation in 
1 out of 200 people. For 110-lb individ- 
uals, this threshold can be expressed as 

I = 116//t (6^), 

where I is the current in milliamperes 
and t is the time in seconds. This re- 
lationship is shown graphically in 
figure 7. The safe area, to the left of 
the plotted line (in figure 7) , is the 
desired region for GFI operation. It 
should be noted that the equation given 
above is only valid for shock durations 
of less than 5 s. 

Procedure 

A variable voltage source in series 
with a 50 ft, 225-W fixed resistance was 
used to inject a 60-Hz current through 
each CT primary as shown in figure 8. A 
double-pole single-throw switch initiated 
the test and triggered the storage oscil- 
loscope. Test currents were varied from 
to 1,000 mA. 

Results 

The test data are listed by manufac- 
turer in tables 1-4. The data were aver- 
aged and plotted with the ventricular 
fibrillation threshold superimposed, as 
shown in figure 9. Statistically, cur- 
rents less than 50 mA should not cause 
ventricular fibrillation. In light of 
this, and since ground currents on a typ- 
ical resistance-grounded system protected 
by a sensitive GFI would be limited to 



E 
III 




50 100 



FIGURE 7. 
at 60 Hz. 



500 1,000 5,000 

CURRENT, mA 

Ventricular fibrillation threshold 




50 100 500 1,000 5,000 
CURRENT, mA 

FIGURE 9. - GFI response at 60 Hz. 



models lacked the sensitivity necessary 
for protection against 50-mA. faults. The 
curves depict relay operation time only. 
To obtain the total clearing time, about 
32 ms should be added, to account for 
opening of the molded-case circuit 
breaker. 



Resistor 
-^WV 



— ^To oscilloscope 
>K ^ trigger 



C 



r/\ Voltmeter 



Current 
I transformer 



•-Current probe 
to oscilloscope 
channel 2 



I'll 



To channel 1 



Relay 

FIGURE 8. - Test setup to determine tripping 
characteristics at 60 Hz. 

500 mA, the area of interest lies between 
50 and 500 mA. 

Close examination revealed that only 
the Mindel GFI's provided complete pro- 
tection against electrocution at 60 Hz. 
However, the older Mindel, with a pick- 
up of 8 mA, was judged to be overly 
sensitive. Both the GE and Harrison 



Power Harmonics 



Rationale 



The filtering for GFI's must be de- 
signed so as to preclude false tripping 
by any harmonics superimposed on the 
power conductors. However, attenuation 
of these higher frequency currents must 
not be so severe that hazardous currents 
are permitted to flow. The ventricular 
fibrillation threshold for humans as a 
function of frequency has been extrapo- 
lated from experiments with animals (7). 
As shown in figure 10, this threshold is 
at a minimum at about 60 Hz and increases 
exponentially with frequency. 

Procedure 

An audio oscillator and power amplifier 
provided high-frequency test currents 
from 60 Hz to 10 kHz, as shown in fig- 
ure 11. For each frequency, the voltage 



TABLE 1. - General Electric: Tripping characteristics at 60 Hz 



I, mA 







Time 


, ms 






Unit 1 


Unit 2 


Unit 3 


Unit 4 


Unit 5 


Unit 6 


ND 


ND 


ND 


ND 


ND 


ND 


ND 


7,200 


5,400 


5,700 


6,000 


6,700 


4,700 


900 


1,000 


1,100 


900 


1,200 


1,000 


800 


350 


500 


350 


750 


100 


180 


120 


100 


200 


250 


70 


40 


40 


40 


50 


50 


30 


25 


23 


30 


40 


25 


20 


10 


15 


25 


30 


15 


20 


10 


13 


25 


25 


20 



100^., 

no.., 

125.., 
150.., 
200.., 
300.., 
500.., 
800.., 
1,000. 



I Current. ND No data; relay did not activate. 
^ Rated sensitivity of General Electric relay. 



TABLE 2. - GBS Harrison: Tripping characteristics at 60 Hz 



I, mA 


Time , ms 




Unit 1 


Unit 2 


Unit 3 


Unit 4 


Unit 5 


Unit 6 


90 


540 
50 
50 
50 
50 
50 
50 
50 
50 
50 
50 
50 


ND 
310 
88 
50 
56 
50 
50 
50 
50 
50 
50 
50 


760 
390 
50 
50 
50 
50 
50 
50 
50 
50 
37 
40 


1,100 
720 
500 
50 
43 
50 
50 
43 
40 
40 
43 
30 


600 
280 
63 
45 
55 
50 
50 
45 
45 
40 
40 
35 


120 


95 


80 


100 


60 


120 


55 


160 


50 


200 


45 


250 


45 


300 

400 


40 
40 


600 


35 


800 


35 


1,000 


35 


I Current . ND 


No data; 


relay c 


id not a 


LCtivate. 







TABLE 3. - Older Mindel: Tripping characteristics at 60 Hz 



I, mA 


Time , ms 


I, mA 


Time , ms 




Unit 1 


Unit 2 


Unit 3 


Unit 4I 


Unit 1 


Unit 2 


Unit 3 


Unit 4I 


10 


ND 


ND 


900 


75 


100 


50 


65 


45 


45 


15 


540 


560 


300 


35 


300 


40 


50 


35 


35 


25 


220 


180 


100 


25 


600 


40 


50 


35 


35 


50 


75 


95 


65 


20 


1,000... 


40 


50 


35 


35 



I Current. ND No data; relay did not activate. 
^Pickup adjusted to maximum setting. 



NOTE. — Units 5 and 6 failed prior to this test. 



TABLE 4. - Newer Mindel: Tripping characteristics at 60 Hz 



50., 
60.. 
75.. 
100. 
150. 



I , mA 



Time , ms 

ND 

2,500 

1,100 

500 

250 



I, mA 



200... 
300... 
500.., 
800... 
1,000. 



Ti 


me , ms 




150 




80 




45 




35 




33 



I Current. ND No data; relay did not activate. 



10 




0.1 I 

FREQUENCY, kHz 
FIGURE 10. - GFI frequency response. 



Current transformer 



Frequency 
counter 



Frequency 
generator 



\®H=^ 



<z> 



FIGURE 1 



Light 
Ammeter ^ 

Frequency-response test circuit. 



attenuated higher frequency currents to 
the extent that ventricular fibrillation 
would be possible. The GE model had no 
filtering and yielded a flat response. 
The frequency characteristics of new Min- 
del units lie close to, but always on the 
safe side of, the fibrillation threshold. 

TRANSIENT IMMUNITY 



was slowly increased until the relay 
activated. 

Results 

The test data are tabulated in tables 
5-8 for each manufacturer, A plot of the 
averaged values for each relay is shown 
superimposed on the allowable attenuation 
curve in figure 10, 

The results indicate that the filtering 
in the Harrison and older Mindel models 



Voltage Surges 

Rationale 

Mine power systems frequently excper- 
ience voltage surges when circuit break- 
ers and switches are opened or closed. 
Although the duration of these tran- 
sients is quite short — a few 60-Hz 
cycles — past research indicates their 
magnitude can reach 5 per unit crest 
voltage at utilization levels (8), These 



11 



TABLE 5. - General Electric: Frequency response data 



Freq, Hz 



60...., 

100 

500 

1,000., 
3,000., 
5,000., 
8,000., 
10,000, 



Unit 1 



112 
100 
80 
80 
80 
82 
80 
100 



Current, mA 



Unit 2 



105 

100 

95 

90 

92 

95 

100 

105 



Unit 3 



105 
100 
90 
90 
90 
90 
95 
95 



Unit 4 



105 
100 
90 
90 
90 
90 
95 
95 



Unit 5 



100 
95 
85 
85 
85 
85 
90 
90 



Unit 6 



100 
95 
85 
85 
85 
90 
95 
95 



TABLE 6. - GBS Harrison: Frequency response data 



Freq, Hz 



60 

100.., 
200.., 
500.., 
800.., 
1,000. 



Current , mA 



Unit 1 



90 

180 

400 

1,000 

1,650 

1,950 



Unit 2 



95 

180 

390 

1,050 

1,600 

2,000 



Unit 3 



90 

170 

350 

1,000 

1,700 

2,000 



Unit 4 



85 

160 

320 

950 

1,500 

1,900 



Unit 5 



90 

180 

380 

1,000 

1,550 

2,000 



Unit 6 



90 

180 

380 

1,050 

1,500 

2,000 










TABLE 7 


. - Older Mindel: 


Frequency 


respons 


e data 






Freq, Hz 


Current , mA 


Freq, Hz 


Current , mA 




Unit 1 


Unit 2 


Unit 3 


Unit 4I 


Unit 1 


Unit 2 


Unit 3 


Unit 4I 


60 

100 

200 


14 
13 
33 


12 
15 

40 


9.5 
10 
28 


10 
13 
31 


500 

1,000... 
3,000... 


130 

420 

1,300 


200 

700 

1,350 


110 

500 

1,350 


100 

450 

1,200 



^Pickup adjusted to maximum setting. 

NOTE. — Units 5 and 6 failed prior to this test. 

TABLE 8. - Newer Mindel: Frequency response data 

Freq, Hz I, mA Freq, Hz 



60 


55 


100 


55 


200 


68 


I Current. 





I, mA 



500 160 

1,000 340 

3,000 1 ,400 



surges , when present on the power conduc- 
tors encircled by the GFI CT, should not 
falsely activate the relay. In addition, 
they should not damage the relay control 
circuitry. 

Procedure 

The impulse tester used to generate 
voltage transients was constructed in 
accordance with Section 19A of Under- 
writers' Laboratories' Standard 943, 



"Ground Fault Circuit Interrupters." 
Consisting of a relay switch and resonant 
circuit, the tester simulates transient 
overvoltages as they would occur on resi- 
dential and industrial power systems. 
Schematics are shown in figures 12 and 
13. 

The generated waveform exhibited the 
following characteristics under no load: 
(1) initial rinse time of 0.5 us between 
10 and 90% of peak amplitude, (2) lO-ps 
period of following oscillatory wave. 



12 



0-9 kVdc 



-WV- 




1-2 

_nTn_ 



CR, 

-O I o- 



_mTi — ,1 



12) 



rfn 




. ^ S-; h 



Relay 
control 



j ITT! 

Cathode roy oscilloscope 
trigger output 



; Device i 
1 under test ! 



3 J 

Neutral ground 



Ci Capacitor 0.025^F; 10 kV 

Cg Copacitor 0.01 ,iF, |0 kV 

Cj Capacitor 4 jlF, 400 v 

L, Coil l5;iH (32 turns. No 23 wire, 

0.7-in-diam air core) 
L2 Coil 70,iH (28 turns. No 23 wire, 

0.6-in-diam air corel 
Tf Transformer l:l, 500 V-A 
S, Switch 



Ri Resistor 22 fl^ I W 

Rg Resistor 12 a. IW 

Rj Resistor 1.3 Mfl(l2xll0kfl, '/2W) 

R4 Resistor 4 7 kQ (10 x 47 kfl, '/2 W) 

/?5 Resistor 200 Q, I/2 W 

CRi Relay 2 normally open poles in series 

GE CR2790 E 100 A2 

F, Fuse 3 A 



FIGURE 12. - Basic impulse generator circuit. 




Cathode ray 

oscilloscope 

gate input.^ 

SCRi 




rm 



KEY 



/?/ Resistor 10 kft, I W 

R2, R3 Resistor I kil, I/2 W 
Ci Capacitor 32 ^F, 250 V 



SCR, 
CR, 



Thyristor GE CI22B 

Relay GE CR2790 E 100 A2 

Transformer Triad N4S X 



Di, Dg Diode 



IN5060 



FIGURE 13. - Relay control circuit for impulse 
generator. 

and (3) amplitude of each successive peak 
60% of the preceding peak. The amplitude 
of the first peak was fully adjustable 
from to 8,000 V. In the first part of 
the test, ten successive 5-kV surges were 
imposed on the power conductors encircled 
by the CT while the relay contacts were 
observed. Next, ten 1-kV impulses were 
applied in parallel with the 120-V ac 
control voltage and at random with re- 
spect to its phase. Afterward, relay op- 
eration at 60 Hz was checked to confirm 
possible circuit damage. 



Results 

Application of the 5-kV surge did not 
affect the GE and Harrison relays. How- 
ever, the Mindel relays were activated 
momentarily following each impulse. This 
may be attributed to their lower pickup. 
All relays operated satisfactorily fol- 
lowing the 1-kV surge to the control 
circuits. 

Common Mode Transients 



Rationale 

Sensitive GFI's used on coal mine power 
systems must be unaffected by the large 
transient currents common to all phases 
of ac utilization circuits. Such cur- 
rents may briefly exceed six times full 
motor rating during startup or heavy in- 
termittent loading. The maximum short- 
circuit settings listed in 30 CFR 75.601 
effectively limit balanced three-phase 
loading to 2,500 A. Nevertheless, bal- 
anced currents up to 2,500 A should be 
tolerated for up to 5 s without actuation 
of the relay. 

Procedure 

The motor test station in the Bureau's 
Pittsburgh, PA, Mine Electrical Labora- 
tory was used to variably supply balanced 
three-phase currents through a trailing 
cable encircled by the GFI CT. The load 
consisted of three 0.3 J2 resistors con- 
nected in a delta configuration. The 
supply voltage was gradually increased 
until the relay activated or the 2,500-A 
ceiling was reached. Tripping thresholds 
were confirmed through repeated tests. 

Results 

Only the Harrison relay was immune to 
common mode transients. All the GE units 
tripped at between 200 and 250 A, while 
the older Mindel GFI's required only 125 
to 200 A to operate. The newer Mindel 
relay did not actuate until 1,700 A was 
reached. 



13 



Current Withstand 

Rationale 

The molded-case circuit breakers used 
on low-voltage ac mine power circuits 
typically have an interrupting rating of 
30,000 A. Currents near this magnitude 
are quite possible for three-phase 
faults. Since the GFI CT is a part of 
the power system, it too must withstand 
up to 30,000 A for the time it takes the 
breaker to clear (a few cycles) . 



High- 
current source 



To storage 
oscilloscope 

1 



Ar 



Shunt 



Mine duty 
circuit breaker 



Current 
\ transformer 



Secondary 
shorted 

FIGURE 14. - Current withstand test circuit. 



Procedure 

The withstand test was conducted using 
a high-current circuit breaker tester, 
as shown in figure 14. The tester was 
equipped with an initiate switch that 
could be jogged to reasonably control the 
test duration. Current magnitudes were 
recorded on a storage oscilloscope con- 
nected across a 400-A, 100-mV shunt. The 
CT secondaries were shorted to preclude 
high secondary voltages. Each was sub- 
jected to 30,000 A for approximately 4 
cycles. The 60-Hz current ratios and 
winding resistances were measured before 
and after the withstand test to detect 
any degradation of the CT. 

Results 

The winding resistance and current 
ratios listed in tables 9 and 10 did 
not change after the withstand test. 
This indicates that all the CT's safely 



tolerated 30,000 A for the time it took 
the breaker to clear. 

RELIABILITY 

Quality Assurance 

Rationale 

For dependability underground, all 
devices made by the same manufac- 
turer should operate in the same manner 
and have a reasonable service life. In 

TABLE 9. - Winding resistance, 9. 



Unit 


GE 


GBS 
Harrison 


Older 
Mindel 


Newer 
Mindel 


1 


0.5 
.6 
.5 
.5 
.5 
.5 


1.3 
1.3 
1.4 
1.4 
1.4 
1.3 


1.2 
1.3 
1.2 
1.3 
1.2 
1.7 


1.3 


2 

3 

4 

5 


NAp 
Nap 
NAp 
NAp 
NAp 


6 



NAp Not applicable. 



TABLE 10. - Current ratio tests (secondary shorted), mA 



Manufacturer 


Primary 






Secon 


dary 








Unit 1 


Unit 2 


Unit 3 


Unit 4 


Unit 5 


Unit 6 


General Electric 


50 
500 


0.13 
3.68 


0.38 
4.02 


0.38 
4.03 


0.38 
4.07 


0.38 
4.06 


0.39 




4.07 


GBS Harrison 


50 

500 


.13 

1.63 


.18 
1.64 


.13 
1.63 


.13 

1.64 


.13 
1.64 


.13 




1.64 


Mindel (older version) 


50 


.71 


.68 


.68 


.71 


.71 


.70 




500 


6.91 


6.92 


6.92 


6.92 


6.92 


6.91 


Mindel (newer version) 


50 


.35 


ND 


ND 


ND 


ND 


ND 




500 


5.02 


ND 


ND 


ND 


ND 


ND 



ND No data. 



14 



addition, each GFI should be equipped 
with a means to test its operation. 

Results 

There was considerable conformity in 
the frequency response data for all 
units. However, the time versus current 
results correlated well only above 150% 
of pickup. The older Mindel relays were 
judged to be unreliable, as two of the 
original six units purchased failed dur- 
ing the test program. Quality assurance 
judgments of the newer version could not 
be made since only one unit was tested. 
All of the relays evaluated featured test 
circuits to simulate ground faults. 



Safe Failure Modes 

Rationale 

In the event of failure of the GFl's 
internal circuitry, the unit should react 
to remove power, to prevent a false sense 
of security. Two common failure modes 
are loss of 120-V ac control power to the 
GFI and opening of the CT winding. 

Results 

Only the GE relays failed to operate 
when control power was removed or the CT 
winding opened. 



SUMMARY AND CONCLUSIONS 



Results of the sensitive GFI tests are 
summarized in table 11. They show that 
none of the commercial three-phase GFI's 
evaluated is suitable for underground 
mining without design modifications. 

Overall, the newer Mindel GFI came 
closest to compliance with the estab- 
lished criteria. Increasing the number 
of secondary turns on the Mindel CT would 



probably eliminate any false tripping 
during motor startups (common mode) . 
False tripping due to voltage surges on 
the power conductors may be eliminated by 

(1) increasing the CT burden impedance or 

(2) changing the rating of the back-to- 
back diode shunt that protects against 
transients coupled through the CT's. The 
unreliability of the older Mindel unit 



TABLE 11, - Summary of results 





General Electric 


GBS Harrison 


Older Mindel 


Newer Mindel 


Military standard com- 


Not corrosion- 


Not corrosion- 


Exposed at 


Passed. 


pliance (terminals). 


resistant. 


resistant. 


120 V ac. 




Mineworthiness 


Lacked metal 


Undersize wire 


Lacked metal 


Lacked metal 




mounting lugs. 


terminals. 


mounting 
lugs. 


mounting 
lugs. 


Proper dimensions 


CT ID too small; 


CT inadequate; 


CT OD and re- 


CT OD too 




relay OK. 


relay OK. 


lay oversized. 


large; relay 
OK. 
Passed. 


Electrocution preven- 


Pickup too high. 


Pickup too 


Too sensitive.. 


tion at 60 Hz. 




high. 






Power harmonics 


May nuisance 


Too much 


Too much 


Do. 




trip. 


attenuation. 


attenuation. 




Voltage surges 


Passed, but 
pickup high. 


Passed, but 
pickup high. 


False tripped. . 


False tripped. 


Common mode 


Failed 


J o 


Failed; too 
sensitive. 


Failed- 










Current withstand 


Passed 


Passed 

...QO.. ....... 


Passed. ........ 


Prl<?^ Pd - 


Quality assurance 


...CO........... 


Failed 


Not 






determined. 


Safe failure modes.... 


Failed 


. . .do. 


Passed 


Passed. 



15 



and its extreme sensltvity preclude its 
use in underground coal mines. 

Both the GE and Harrison GFI's require 
many modifications to pass the tests. 
Both must be made more sensitive to de- 
tect 50 mA faults. The GE CT should be 
wound regressively for common mode immu- 
nity. The filtering of the Harrison re- 
lay should be modified to detect hazard- 
ous high-frequency currents. 



The results of these tests are intended 
to serve as a basis for the development 
of a mineworthy, sensitive GFI in ongoing 
Bureau research. Such a device could 
be expected to prevent nearly all elec- 
trocutions on coal mine ac utilization 
circuits. 



REFERENCES 



1. Cooley, W. L. , B. S, Tenney, and 
Z. Elrazaz. Analysis of Coal Mine Elec- 
trical Accidents (contract J010096, WV 
Univ.). BuMines OFR 91-82, 1981, 242 
pp.; NTIS PB 82-244740. 

2. Morley, L. A., F. C. Trutt, and 
D, J. Rufft. Electrical-Shock Prevention 
(contract JOl 13009, PA State Univ.). 
Volume 2 — Ground-Fault Interrupting De- 
vices. BuMines OFR 177(2)-83, 1982, 110 



pp, 



NTIS PB 84-102953. 



R. D. Evans. 
McGraw-Hill, 



3. Wagner, C. F., and 
Symmetrical Components. 
1933, 437 pp. 

4. U.S. Navy. U.S. Standard Require- 
ments for Electrical Equipment. Military 
Standard 454J, Sec. 1 and 36, Apr. 1984, 
pp. 1-1 through 1-12 and 36-1 and 36-2. 

5. Anaconda-Ericson Co. Mining Cable 
Engineering Handbook. Wire and Cable 
Div., Greenwich, CT, 1976, 168 pp. 



6. Dalziel, C. F., and W. R. Lee, Re- 
evaluation of Lethal Electric Current. 
IEEE Trans., Ind. Gen. Appl. , v. IGA-4, 
No. 5, 1968, p. 467. 

7. Geddes, L. A., and L. E. Baker. 
Response to Passage of Electric Current 
Through the Body. J. Assoc. Adv. Med. 
Instru. , V. 5, No. 1, Jan. -Feb. 1971, 
p. 14. 

8. Stanek, E. K. , W. Vilcheck, and 
A. Kunjara. Mine Electrical Power Sys- 
tems. Tansients Protection, Reliabil- 
ity Investigation, and Safety Testing of 
Mine Electrical Power Systems (grant 
GO 144137, WV Univ.). Volume l~Tran- 
sients in Mine Electrical Power Systems. 
BuMines OFR 6(1)-81, 1979, 169 pp.; NTIS 
PB 81-166761. 



*U.S. GPO: 1985-605-017/20,132 



IN T.-BU.O F MIN ES,PGH.,P A. 28155 
















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